Imagine a wind turbine, and the chances are that you see three blades mounted on a horizontal shaft at the top of a tall tower. This type of turbine is the most common, but many others exist. Some have six or more blades, and some even have more than one rotor. These are all types of horizontal axis wind turbine (HAWT).
You may also have seen vertical axis wind turbines (VAWTs), particularly in built-up areas or near roads. These are distinctive turbines that generally have a smaller tower, and blades rotating around a vertical shaft. There are several types of these, recognisable as being “H” shaped, or resembling egg beaters, or a mixture of the two.
From observations of existing wind power developments, it seems the market has decided firmly in favour of HAWTs. HAWTs form the vast majority of wind power installations, and are the default choice as their years of proven performance are attractive to investors.
What is the reason for HAWTs’ dominance? At the moment, HAWTs offer better performance, giving a higher yield of electricity. This is particularly advantageous for large-scale projects with excellent wind resources, but at smaller scales or with poorer wind resources, VAWTs may offer some real advantages.
VAWTs have had a reputation for poor reliability. This is for several reasons. They produce higher torque, which places more stress on the drivetrain. This can be addressed using an appropriate drivetrain. Cyclical stress is applied to the tower as the blades rotate around it, which also produces “torque ripple”, although this can be addressed by the shape and configuration of the blades.
Historically, VAWTs have had problems with fatigue in their blades, although this has been addressed by using composite blades rather than aluminium.
In fact, VAWTs could offer greater reliability than HAWTs. They do not need yawing mechanisms, as they do not need to turn in to the wind. They place lower loads on their supporting structure – either the foundations or the building, if they are building-mounted. Plus, they have fewer moving parts and could be mechanically simpler.
Finally, the drive train and generator are at ground level, which gives easy access for maintenance. In HAWTs these components are at the top of the tower, making maintenance more difficult, time consuming and expensive.
Noise from wind turbines can be a problem, particularly when they are close to occupied buildings. There are two components to turbine noise: aerodynamic noise and mechanical noise.
Aerodynamic noise is caused by the blades “swooshing” through the air, and the level of noise is related to the tip speed – the speed at which the tips of the blades are travelling. Tip speed is much lower for VAWTs than HAWTs, giving less noise. HAWTs can also produce a “thumping” noise as each blade passes the tower – VAWTs do not produce this.
Mechanical noise is produced by the drivetrain, and the gearbox in turbines that have one. Mechanical noise from VAWTs is not propagated as far, because these components are at ground level.
Turbulence is caused by obstacles such as buildings, trees, tall crops and sudden changes in topography, for example a sharp ridge or cliff edge. Turbulence has a particularly detrimental effect on HAWTs as it reduces their energy output, and can also cause fatigue and premature failure.
Turbulence is generally high in built-up areas, and wind speeds amongst buildings are unpredictable and generally low. For this reason turbines are generally located away from urban areas and buildings.
VAWTs are less badly affected by turbulence, and can successfully operate on high turbulence sites. Because they place lower loads on their support structures, they can be mounted on tall buildings, giving them access to better wind resources.
Their lower noise characteristics also make them more suitable for more built-up environments.
There is some confusion surrounding the term efficiency when it comes to wind turbines. Figures of 20-30% are often quoted, but these do not represent efficiency, they are a different parameter called “capacity factor” (see below). A more useful measure of a wind turbine’s performance is power coefficient, or Cp. This is a measure of what proportion of the wind energy arriving at the rotor is converted to electrical energy.
The theoretical limit for Cp is 59%. Exact figures are scarce, but suggest that HAWTs are approaching this figure, at around 40-50%; VAWTs lag behind at around 20%.
This is demonstrated by manufacturers’ claimed figures. For example, one MCS-certified HAWT claims a capacity factor of 28%, if average wind speed is 7m/s. A similar sized, MCS-certified VAWT claims a capacity factor of 17% at the same wind speed. MCS-certified turbines have been used for the comparison, because manufacturers of turbines that are not MCS-certified can make misleading claims about their performance.
HAWTs in conventional wind farms must be spaced far enough apart to avoid the wake of one turbine affecting the performance of turbines downwind of it. This spacing may be six-times the rotor diameter or more. Spacing turbines far apart greatly reduces the amount of power that can be produced from an area of land – “power density”.
Greater power is produced by using taller turbines, with larger rotor areas and access to higher wind speeds. It is possible, however, that tightly-packed arrays of VAWTs could produce much higher power densities, without the need for very large turbines. Modern HAWT arrays may produce 2-3W per square metre, but research by Caltech (California institute of Technology) suggests that VAWT arrays could produce power densities much greater than this, greatly reducing the visual impact, environmental impacts and cost of wind farms.
This effect is not relevant to projects using a single turbine in isolation. In fact, farmers using a single VAWT may find that it uses more space than an equivalent HAWT, as some models have guy wires which dramatically increase the turbine’s footprint.
The largest turbine in production is a HAWT – the Enercon E126, with a rated power of 7.58MW and tip height of 198m. Engineering constraints make it extremely difficult to keep building bigger HAWTs. The largest operational VAWT in the world has a rated power of 4MW and is 110m tall; however, a completely new type of giant VAWT is in development, which could have a rated power of 10MW.
It is clear that for the moment, HAWTs tend to offer higher performance and higher yield, making them the turbine of choice for open sites with good wind resources. VAWTs do offer clear advantages in terms of tolerating turbulence, producing lower noise levels and offering easier access to the drivetrain and generator. These advantages point to use in urban and built-up sites, and sites where high turbulence is inevitable, such as sharp ridges.
In the future, the performance of VAWTs may be improved, leading to the use of very large VAWTs, and densely packed clusters of medium-scale VAWTs.
Charles Romijn is a project engineer on the IsWindTech project at the University of Central Lancashire. It offers free comprehensive and impartial feasibility studies to businesses in the northwest considering a wind turbine. Go to www.uclan.ac.uk/iswindtech to find out more